Project Summary Proper localization of proteins is crucial for all cells. The signal recognition particle (SRP) and its receptor (SR) constitute the major cellular machinery that mediates the co-translational targeting of roughly one third of cellular proteins to the eukaryotic endoplasmic reticulum, or the bacterial plasma membrane. Although enormous progress has been made in understanding the simplest SRP in bacteria, eukaryotic SRP is four times larger and contains five additional protein subunits. Compared to bacterial SRP, our understanding of the more complex mammalian SRP lags far behind. In this grant cycle, we will establish a rigorous mechanistic framework for the mammalian SRP pathway and decipher the role of eukaryote-specific components in this particle. We will interrogate the molecular mechanisms by which the SRP receptor interacts with and exerts regulatory functions at the membrane in both the bacterial and mammalian SRP pathways. Understanding the mammalian SRP pathway will likely unravel new layers of regulation in higher eukaryotic cells. Comparison between the bacterial and mammalian SRP will reveal general principles that enable the balance between efficiency and accuracy in both organisms, and shed light on the interplay between protein and RNA during the evolution of ancient ribonucleoprotein particles. The proposed research is of a most basic nature, and will contribute profoundly to our general understanding of physiology and pathology of all living cells at the molecular level.